Various arrangements utilizing multi-reflection to extend the flight path of ions within mass spectrometers are known. Flight path extension is desirable to increase time-of-flight separation of ions within time-of-flight (TOF) mass spectrometers or to increase the trapping time of ions within electrostatic trap (EST) mass spectrometers. In both cases the ability to distinguish small mass differences between ions is thereby improved. Improved resolving power, along with advantages in increased mass accuracy and sensitivity that typically come with it, is an important attribute for a mass spectrometer for a wide range of applications, particularly with regard to applications in biological science, such as proteomics and metabolomics for example.
An arrangement of two parallel opposing mirrors was described by Nazarenko et. al. in patent SU1725289. These mirrors were elongated in a drift direction and ions followed a zigzag flight path, reflecting between the mirrors and at the same time drifting relatively slowly along the extended length of the mirrors in the drift direction. Each mirror was made of parallel bar electrodes. The number of reflection cycles and the mass resolution achieved were able to be adjusted by altering the ion injection angle. The design was advantageously simple in that only two mirror structures needed to be produced and aligned to one another. However this system lacked any means to prevent beam divergence in the drift direction. Due to the initial angular spread of the injected ions, after multiple reflections the beam width may exceed the width of the detector making any further increase of the ion flight time impractical due to the loss of sensitivity. Ion beam divergence is especially disadvantageous if trajectories of ions that have undergone a different number of reflections overlap, thus making it impossible to detect only ions having undergone a given number of oscillations. As a result, the design has a limited angular acceptance and/or limited maximum number of reflections. Furthermore, the ion mirrors did not provide time-of-flight focusing with respect to the initial ion beam spread across the plane of the folded path, resulting in degraded time-of-flight resolution for a wide initial beam angular divergence.
Wollnik, in GB patent 2080021, described various arrangements of parallel opposing gridless ion mirrors. Two rows of mirrors in a linear arrangement and two opposing rings of mirrors were described. Some of the mirrors may be tilted to effect beam injection. Each mirror was rotationally symmetric and was designed to produce spatial focusing characteristics so as to control the beam divergence at each reflection, thereby enabling a longer flight path to be obtained with low beam losses. However these arrangements were complex to manufacture, being composed of multiple high-tolerance mirrors that required precise alignment with one another. The number of reflections as the ions passed once through the analyser was fixed by the number of mirrors and could not be altered.
Su described a gridded parallel plate mirror arrangement elongated in a drift direction, in International Journal of Mass Spectrometry and Ion Processes, 88 (1989) 21-28. The opposing ion reflectors were arranged to be parallel to each other and ions followed a zigzag flight path for a number of reflections before reaching a detector. The system had no means for controlling beam divergence in the drift direction, and this, together with the use of gridded mirrors which reduced the ion flux at each reflection, limited the useful number of reflections and hence flight path length.
Verentchikov, in WO2005/001878 and GB2403063 described the use of periodically spaced lenses located within the field free region between two parallel elongated opposing mirrors. The purpose of the lenses was to control the beam divergence in the drift direction after each reflection, thereby enabling a longer flight path to be advantageously obtained over the elongated mirror structures described by Nazarenko at al. and Su. To further increase the path length, it was proposed that a deflector be placed at the distal end of the mirror structure from the ion injector, so that the ions may be deflected back through the mirror structure, doubling the flight path length. However the use of a deflector in this way is prone to introducing beam aberrations which would ultimately limit the maximum resolving power that could be obtained. In this arrangement the number of reflections is set by the position of the lenses and there is not the possibility to change the number of reflections and thereby the flight path length by altering the ion injection angle. The construction is also complex, requiring precise alignment of the multiple lenses. Lenses and the end deflector are furthermore known to introduce beam aberrations and ultimately this placed limits on the types of injection devices that could be used and reduced the overall acceptance of the analyser. In addition, the beam remains tightly focused over the entire path making it more susceptible to space charge effects.
Makarov et. al., in WO2009/081143, described a further method of introducing beam focusing in the drift direction for a multi-reflection elongated TOF mirror analyser. Here, a first gridless elongated mirror was opposed by a set of individual gridless mirrors elongated in a perpendicular direction, set side by side along the drift direction parallel to the first elongated mirror. The individual mirrors provided beam focusing in the drift direction. Again in this arrangement the number of beam oscillations within the device is set by the number of individual mirrors and cannot be adjusted by altering the beam injection angle. Whilst less complex than the arrangement of Wollnik and that of Verentchikov, nevertheless this construction is more complex than the arrangement of Nazarenko et. al. and that of Su.
Golikov, in WO2009001909, described two asymmetrical opposed mirrors, arranged parallel to one another. In this arrangement the mirrors, whilst not rotationally symmetric, did not extend in a drift direction and the mass analyzer typically has a narrow mass range because the ion trajectories spatially overlap on different oscillations and cannot be separated. The use of image current detection was proposed.
A further proposal for providing beam focusing in the drift direction in a system comprising elongated parallel opposing mirrors was provided by Verentchikov and Yavor in WO2010/008386. In this arrangement periodic lenses were introduced into one or both the opposing mirrors by periodically modulating the electric field within one or both the mirrors at set spacings along the elongated mirror structures. Again in this construction the number of beam oscillations cannot be altered by changing the beam injection angle, as the beam must be precisely aligned with the modulations in one or both the mirrors. Each mirror is somewhat more complex in construction than the simple planar mirrors proposed by Nazarenko et. al.
A somewhat related approach was proposed by Ristroph et. al. in US2011/0168880. Opposing elongated ion mirrors comprise mirror unit cells, each having curved sections to provide focusing in the drift direction and to compensate partially or fully for a second order time-of-flight aberration with respect to the drift direction. In common with other arrangements, the number of beam oscillations cannot be altered by changing the beam injection angle, as the beam must be precisely aligned with the unit cells. Again the mirror construction is more complex than that of Nazarenko et. al.
All arrangements which maintain the ions in a narrow beam in the drift direction with the use of periodic structures necessarily suffer from the effects of space-charge repulsion between ions.
Sudakov, in WO2008/047891, proposed an alternative means for both doubling the flight path length by returning ions back along the drift length and at the same time inducing beam convergence in the drift direction. In this arrangement the two parallel gridless mirrors further comprise a third mirror oriented perpendicularly to the opposing mirrors and located at the distal end of the opposing mirrors from the ion injector. The ions are allowed to diverge in the drift direction as they proceed through the analyser from the ion injector, but the third ion mirror reverts this divergence and, after reflection in the third mirror, upon arriving back in the vicinity of the ion injector the ions are once again converged in the drift direction. This advantageously allows the ion beam to be spread out in space throughout most of its journey through the analyser, reducing space charge interactions, as well as avoiding the use of multiple periodic structures along or between the mirrors for ion focusing. The third mirror also induces spatial focussing with respect to initial ion energy in the drift direction. There being no individual lenses or mirror cells, the number of reflections can be set by the injection angle. However, the third mirror is necessarily built into the structure of the two opposing elongated mirrors and effectively sections the elongated mirrors, i.e. the elongated mirrors are no longer continuous—and nor is the third mirror. This has the disadvantageous effect of inducing a discontinuous returning force upon the ions due to the step-wise change in the electric field in the gaps between the sections. This is particularly significant since the sections occur near the turning point in the drift direction where the ion beam width is at its maximum. This can lead to uncontrolled ion scattering and differing flight times for ions reflected within more than one section during a single oscillation.
Recently, US2015/0028197 described a multi-reflection mass spectrometer comprised of two ion mirrors, opposing each other in the X direction and both being generally elongated in the drift direction Y. Ions injected into the instrument are repeatedly reflected back and forth in X direction between the mirrors, whilst they drift down the Y direction of mirror elongation. Overall, the ion motion follows a zigzag path. The mirrors have a convergence with increasing Y, thereby creating a pseudo-potential gradient along the Y axis that acts as an ion mirror to reverse the ion drift velocity along Y and spatially focus the ions in Y to a focal point where a detector is placed. Thus, the pseudo-potential gradient along the Y axis enables the ion motion to be reversed without actually requiring a third ion mirror as described in Sudakov.
In view of the above, however, improvements are still desired, for example in resolving power.